Light emitting diodes (hereinafter also referred to as LED chips) are light emitting devices (light emitting elements) that act as light sources upon application of a voltage and utilize light emitted as a result of recombination between electrons and positive holes in the vicinity of a contact surface (pn-junction) between two semiconductors. These light emitting devices are small in size and have a high conversion efficiency of electric energy into light, and therefore are widely used as household electrical appliances, lighted operation switches, and LED indicators (LED displays).
Differing from electric lamp bulbs using filaments, the light emitting diodes are semiconductor devices, are thereby free from blowout, are excellent in initial drive performance, and have excellent durability even under vibrations and/or repeated ON/OFF operations. They are therefore also used as backlights of indicators or displays typically for automobile dashboards. Particularly, since they can emit light of a clear color with high color saturation without being affected by sunlight, the uses of the light emitting diodes will be expanded even to, for example, displays arranged outdoor, displays for traffic use, and traffic signals, or the like.
As conventional light emitting apparatuses bearing light emitting devices such as LED chips, for example, there is proposed a light emitting apparatus shown in FIG. 4 (see, for example, Patent Document (Japanese Unexamined Patent Application Publication No. 10-215001)). The light emitting apparatus 1 comprises a ceramic package 3, a LED chip as a light emitting device, a first metal layer 6, a second metal layer 7, and a resin molding 8. The ceramic package 3 includes conductive interconnections 2 and has a concave opening. The LED chip 5 is electrically connected to the conductive interconnections 2 via bonding wires 4 in the concave opening. The first metal layer 6 and the second metal layer 7 are arranged on side walls of the concave opening. The resin molding 8 seals the concave opening.
The patent document mentions that, according to the conventional light emitting device, the first metal layer 6 arranged in the concave opening acts to increase the adhesion with the ceramic package 3, and, additionally, the second metal layer 7 acts to reflect light emitted from the LED chip 5, the light loss can thereby be reduced and the contrast typically in displays can be increased.
The conventional light emitting apparatus, however, has a fatal defect of very poor heat radiation performance, since the ceramic package bearing the LED chip comprises a ceramic material mainly containing alumina (Al2O3) and having a low thermal conductivity, and the molding resin for sealing the LED chip also has a low thermal conductivity. The LED chip may be broken due to heat generated upon application of a high voltage and/or a large current. Consequently, the conventional light emitting apparatus has a low luminance, since the highest voltage that can be applied to the LED chip is low and the current to be supplied is limited to several ten milliamperes.
Since only a low luminance is required, the conventional light emitting apparatus using a LED chip has been practically used without significant problems even at the above-mentioned current quantity. With recent expanding specific uses of LED light emitting apparatuses, however, technical demands have been made to achieve structures that can increase the current to be passed to about several amperes at a higher power and can thereby increase the luminance.
Additionally, in conventional light emitting apparatuses as shown in FIG. 4, the LED chip and the conductive interconnections are electrically connected by a wire bonding process, so that a portion where the bonding wire rises protrudes in a thickness direction of the apparatus, and a large electrode region for connecting the edge of the bonding wire is disadvantageously required. Thus, there has been posed a problem that the LED package including the interconnection structure becomes large in size.
Furthermore, when the LED chip is mounted and housed in a concave opening as shown in FIG. 4 so as to avoid the adverse influence of the bonding wire protruding in a thickness direction of the apparatus, the light emitted from the LED chip is absorbed by the inner wall of the concave opening to increase the light loss and thereby to decrease the luminous efficiency. Thus, according to the conventional technique, a metal layer that reflects light is arranged on the inner wall of the concave opening to thereby reduce the absorption loss of light. However, it is very difficult to form such a reflecting metal layer uniformly in the concave opening having a curved inner wall, and the emitted light is partially absorbed by the inner wall to invite light loss. In addition, there has been also posed another problem that the inner wall of the concave opening itself has such a structure as to inhibit the travel or transmission of the light, and the luminance is thereby decreased.
The present invention has been achieved to solve the conventional problems, and an object of the present invention is to provide a light emitting apparatus that can be reduced in size, is excellent in heat radiation performance, allows a larger current to pass therethrough, and can have a significantly increased luminance with a high luminous efficiency.